1. Field of the Invention
The invention relates to a cooling system for cooling the recirculated exhaust gases of an internal combustion engine.
2. Description of Related Art
Stringent exhaust gas emission regulations for internal combustion engines, especially diesel engines, have led to the development of new emission-reducing technologies. One such technology is the cooled exhaust gas recirculation ("EGR") method which reduces emissions with only a slight increase in fuel consumption. The heat exchanger of an EGR system must be capable of meeting high demands in terms of compact design, performance, and resistance to high temperatures, corrosion and fouling.
In a known EGR system, as shown in FIG. 1, a turbocharged diesel engine 1 has an exhaust gas cooler (heat exchanger) 2, a radiator 3 and a charge air cooler 4. Outside air is drawn into the intake system through the compressor 5 of an exhaust-driven turbocharger having a turbine 7. Exhaust gas emitted from the diesel engine 1 passes through a valve 6 that controls the amount of exhaust gas that flows through the remainder of the exhaust system, including turbine 7, to the ambient.
Valve 6 also controls the amount of exhaust gas that is recirculated and mixed with the charge air. The exhaust gas that enters the EGR system flows through the exhaust gas cooler 2, where it transfers heat to the engine coolant circulating in the engine cooling system loop 9 by means of pump 8. After the exhaust gas has been cooled in the exhaust gas cooler 2, it is admixed at 6a with cooled intake air prior to entering the diesel engine.
The coolant circulating in the engine coolant loop 9 has already absorbed combustion heat from engine 1 when it arrives at exhaust gas cooler 2. In exhaust gas cooler 2 additional heat is transferred to the coolant from the exhaust gas. The hot coolant then flows via pump 8 to radiator 3 where heat is rejected to the ambient air. Because the coolant carries more heat from the exhaust gas cooler 2 than it would otherwise carry if it were only cooling the engine 1, more heat must be rejected in the radiator 3. This requires an increase in the frontal area of the radiator 3. In present commercial vehicles, however, the radiator already occupies the available frontal area. A further increase in the frontal area thus is not possible.
Another cooling system is disclosed in U.S. Pat. No. 5,607,010, in which at least two heat exchangers are serially arranged to cool the exhaust gases in an EGR system for a diesel engine. The cooling of the exhaust gases thus takes place in stages, at progressively lower operating temperature ranges. The serial arrangement allows each heat exchanger to be optimally tailored for its own temperature-specific cooling task, thus preventing contamination and/or corrosion from forming in the heat exchangers. This patent, however, does not describe the coolant circuit(s) for these heat exchangers, i.e., whether or how they may relate to each other or to the cooling system for the engine, or the location of the radiators. As noted above, in present day commercial vehicles, the entire frontal area is already occupied by a single radiator and thus an increased frontal radiator area is not possible.